Commercial desulfurization techniques only remove a portion of the sulfur present in chars. This is inadequate for purposes of complying with air purity standards which limit the amount of sulfur dioxide produced by the use of chars as a fuel. The need for a relatively sulfur-free char has therefore become very important, especially in view of dwindling supplies of oil and natural gas and abundant supplies of coal.
Conventional commercial methods for desulfurizing char generally attempt to desulfurize the carbonaceous material from which the char is manufactured, such as coal, rather than treating the char itself. Sulfur is more readily removed from coal than char because the conversion to char causes reactions between the sulfur and the inorganic and organic compounds in the coal which "fixes" the sulfur in the resultant char thereby making it much more difficult to remove.
One method for desulfurizing coal is by crushing and grinding the coal into fines which are introduced into a fluid medium to allow the sulfur in the form of FeS.sub.2 (pyrite) to form a separate layer which is mechanically removed from the coal. This technique, however, does not remove the FeS.sub.2 embedded in the uncrushed coal and does not remove any of the sulfur present in other inorganic compounds and in organic compounds.
Another method is to treat coal with hydrogen gas to leach out the sulfur by reacting therewith to form hydrogen sulfide gas.
A hot molten mixture of sodium and potassium hydroxide has also been used to leach sulfur from finely divided coal. The art discloses that the sulfur is leached out by the molten caustic only after the coal is in a semi-fluid or "plastic" condition. Char, however, does not become semi-fluid or plastic.